Endoscope system

ABSTRACT

An endoscope system includes an imaging element, a voltage-current conversion circuit, a first coaxial cable, and an impedance conversion circuit. The imaging element generates a first voltage. The voltage-current conversion circuit is disposed inside or outside the imaging element and converts the first voltage into a first current. The first coaxial cable has a first conductor and a second conductor. The second conductor is disposed outside the first conductor. The first current is transmitted through the first conductor. The first current transmitted through the first conductor is input to the impedance conversion circuit. The impedance conversion circuit outputs a second current according to the first current. A second voltage according to the first current is input to the second conductor.

This application is a continuation application based on InternationalPatent Application No. PCT/JP2015/081596 filed on Nov. 10, 2015, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an endoscope system.

Description of Related Art

Endoscope devices having a scope are widely used. The scope has anelongated insertion portion. By inserting the insertion portion into abody cavity, a user can observe organs in the body cavity.Alternatively, by inserting a treatment tool into a treatment toolchannel, the user can perform various treatment procedures. Also inindustrial fields, industrial endoscope devices having a scope arewidely used. By using an industrial endoscope device, a user can observeor inspect internal scratches and corrosion of a boiler, a turbine, anengine, a chemical plant and so on.

Generally, in a scope (electronic endoscope), a solid state imagingdevice as an imaging element is mounted at a distal end of the insertionportion. For example, the solid state imaging device is a charge coupleddevice (hereinafter, referred to as CCD). A signal line which transmitsa driving signal for operating the CCD and a signal line which transmitsan imaging signal from the CCD are disposed inside the insertionportion.

A scope (electronic endoscope) has a long insertion portion. A length ofthe signal line disposed inside the insertion portion is proportional toa length of the insertion portion. Therefore, the signal line may becomelong. When the signal line is long, the signal from the CCD is easilyattenuated. To stabilize a waveform of the signal, a coaxial cable isgenerally used. The coaxial cable has a shield structure which preventselectrical interference and noise influence with respect to the signal.For example, the coaxial cable has a first conductor and a secondconductor. The first conductor is disposed at the center of the coaxialcable. The second conductor covers the first conductor and is insulatedfrom the first conductor.

In Japanese Unexamined Patent Application, First Publication No.H7-184854, an endoscope system is disclosed in which a bundle-shapedsignal line is disposed. The bundle-shaped signal line includes a firstsignal line having a large diameter for preventing attenuation of asignal and a second signal line having a diameter different from that ofthe first signal line. Each of the signal lines is constituted with acoaxial cable.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, an endoscopesystem includes an imaging element, a voltage-current conversioncircuit, a first coaxial cable, and an impedance conversion circuit. Theimaging element generates a first voltage. The voltage-currentconversion circuit is disposed inside or outside the imaging element andconverts the first voltage into a first current. The first coaxial cablehas a first conductor and a second conductor. The second conductor isdisposed outside the first conductor. The first current is transmittedthrough the first conductor. The first current transmitted through thefirst conductor is input to the impedance conversion circuit. Theimpedance conversion circuit outputs a second current according to thefirst current. A second voltage according to the first current is inputto the second conductor.

According to a second aspect of the present invention, in the firstaspect, the endoscope system may further include a buffer configured tooutput the second voltage according to the first current.

According to a third aspect of the present invention, in the firstaspect, the endoscope system may further include a second coaxial cableconfigured to transmit a power source voltage supplied to the imagingelement.

According to a fourth aspect of the present invention, in the firstaspect, the endoscope system may further include a current-voltageconversion circuit and a circuit board. The current-voltage conversioncircuit may convert the second current output from the impedanceconversion circuit into a third voltage. The impedance conversioncircuit and the current-voltage conversion circuit may be disposed onthe circuit board. The circuit board may include a first terminal and asecond terminal. The first current transmitted through the firstconductor may be input to the first terminal. The first terminal may beelectrically connected to the impedance conversion circuit. The secondterminal may be electrically connected to the impedance conversioncircuit and may output the second voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a constitution of an endoscope systemaccording to a first embodiment of the present invention.

FIG. 2 is a block diagram showing the constitution of the endoscopesystem according to the first embodiment of the present invention.

FIG. 3 is a cross-sectional view of a cable used in the endoscope systemaccording to the first embodiment of the present invention.

FIG. 4 is a block diagram showing a constitution of a main part of theendoscope system according to the first embodiment of the presentinvention.

FIG. 5 is a block diagram showing a constitution of a main part of anendoscope system according to a modified example of the first embodimentof the present invention.

FIG. 6 is a block diagram showing a constitution of a main part of anendoscope system according to a second embodiment of the presentinvention.

FIG. 7 is a block diagram showing a constitution of a main part of anendoscope system according to a third embodiment of the presentinvention.

FIG. 8 is a block diagram showing a constitution of a main part of anendoscope system according to a modified example of the third embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention will be described withreference to the drawings.

First Embodiment

FIG. 1 is a schematic view showing a constitution of an endoscope system1 according to a first embodiment of the present invention. As shown inFIG. 1, the endoscope system 1 includes a scope 2, a transmission cable3, an operation unit 4, a connector unit 5, a processor 6, and a displaydevice 7.

The scope 2 has an insertion portion 70 which is inserted into asubject. The insertion portion 70 is a part of the transmission cable 3.The insertion portion 70 is inserted into the subject. The scope 2generates an imaging signal (image data) by taking an image inside thesubject. The scope 2 outputs the generated imaging signal to theprocessor 6. An imaging unit 20 shown in FIG. 2 is disposed at a distalend 71 of the insertion portion 70. In the insertion portion 70, theoperation unit 4 is connected to an end thereof on the side opposite tothe distal end 71. The operation unit 4 receives various operations onthe scope 2.

The transmission cable 3 connects the imaging unit 20 of the scope 2 andthe connector unit 5. The imaging signal generated by the imaging unit20 is output to the connector unit 5 via the transmission cable 3.

The connector unit 5 is connected to the scope 2 and the processor 6.The connector unit 5 performs a predetermined signal processing on theimaging signal output from the scope 2. Further, the connector unit 5performs an A/D conversion of an analog imaging signal into a digitalsignal. The connector unit 5 outputs an imaging signal which is adigital signal to the processor 6.

The processor 6 performs a predetermined image processing on the imagingsignal output from the connector unit 5 and generates a picture signal.Also, the processor 6 totally controls the entire endoscope system 1.

The display device 7 displays an image corresponding to the picturesignal processed by the processor 6. Further, the display device 7displays a variety of information on the endoscope system 1.

The endoscope system 1 has a light source device which generatesillumination light to be irradiated on a subject. In FIG. 1, the lightsource device is omitted.

FIG. 2 shows an internal constitution of the endoscope system 1. Asshown in FIG. 2, the endoscope system 1 includes the imaging unit 20,the transmission cable 3, the connector unit 5, and the processor 6.

The imaging unit 20 has a first chip 21 and a second chip 22. The firstchip 21 includes a light receiving unit 23, a reading unit 24, a timinggenerating unit 25, and a buffer 26. The imaging unit 20 serves as animaging element. The imaging unit 20 outputs an imaging signal. Theimaging unit 20 includes the light receiving unit 23 in which aplurality of pixels outputting a first voltage are disposed. The firstvoltage is a voltage of the imaging signal.

The light receiving unit 23 has the plurality of pixels and generatesthe imaging signal based on incident light. The reading unit 24 readsthe imaging signal generated by the light receiving unit 23. Also, thereading unit 24 generates a reference signal. The timing generating unit25 generates a timing signal on the basis of a reference clock signaland a synchronization signal output from the connector unit 5. Thetiming signal generated by the timing generating unit 25 is output tothe reading unit 24. The reading unit 24 reads the imaging signalaccording to the timing signal. The buffer 26 temporarily holds theimaging signal read out from the light receiving unit 23 and temporarilyholds the reference signal. The first chip 21 outputs the imaging signalfrom the buffer 26.

The second chip 22 has a buffer 27. The buffer 27 outputs the imagingsignal output from the first chip 21 to the connector unit 5 via thetransmission cable 3. The imaging signal is input to the buffer 27 as aninput signal VIN. A combination of circuits mounted on the first chip 21and the second chip 22 can be appropriately changed according to thedesign. In the endoscope system 1 shown in FIG. 1, the buffer 27 isdisposed inside the imaging unit 20. The buffer 27 may be disposedinside the scope 2 and outside the imaging unit 20. As described above,the scope 2 has the imaging unit 20 and the buffer 27.

A power source voltage generated by the processor 6 and a ground voltageare transmitted to the imaging unit 20 by the transmission cable 3. Inthe imaging unit 20, a power supply stabilizing capacitor C100 isdisposed between a signal line which transmits the power source voltageand a signal line which transmits the ground voltage.

The connector unit 5 includes an analog-front-end unit 51 (hereinafterreferred to as an AFE unit 51), a preprocessing unit 52, and a controlsignal generating unit 53. The connector unit 5 electrically connectsthe scope 2 (imaging unit 20) and the processor 6. The connector unit 5and the imaging unit 20 are connected by the transmission cable 3. Theconnector unit 5 and the processor 6 are connected by a coil cable.

The AFE unit 51 (imaging signal processing circuit) calculates adifference between the reference signal and the imaging signal. Further,the AFE unit 51 performs an A/D conversion of the imaging signal basedon the difference. The AFE unit 51 outputs the imaging signal convertedinto the digital signal by the A/D conversion to the preprocessing unit52. The AFE unit 51 includes a circuit board 510. The circuit board 510includes a circuit which processes the imaging signal. A constitution ofthe circuit board 510 will be described later.

The preprocessing unit 52 performs a predetermined signal processingsuch as vertical line removal and noise removal on the digital imagingsignal output from the AFE unit 51. The preprocessing unit 52 outputsthe imaging signal subjected to the signal processing to the processor6.

The reference clock signal serving as a reference for an operation ofeach portion of the scope 2 is supplied from the processor 6 to thecontrol signal generating unit 53. For example, a frequency of thereference clock signal is 27 MHz. The control signal generating unit 53generates a synchronization signal indicating a start position of eachframe on the basis of the reference clock signal. The control signalgenerating unit 53 outputs the reference clock signal and thesynchronization signal to the timing generating unit 25 of the imagingunit 20 via the transmission cable 3. The synchronization signalgenerated by the control signal generating unit 53 includes a horizontalsynchronization signal and a vertical synchronization signal.

The processor 6 is a control device which totally controls the entireendoscope system 1. The processor 6 includes a power source unit 61, apicture signal processing unit 62, and a clock generating unit 63.

The power source unit 61 generates the power source voltage. The powersource unit 61 outputs the power source voltage and the ground voltageto the imaging unit 20 via the connector unit 5 and the transmissioncable 3.

The picture signal processing unit 62 (picture signal generatingcircuit) performs a predetermined image processing on the digitalimaging signal processed by the preprocessing unit 52. The predeterminedimage processing includes a synchronizing processing, a white balance(WB) adjusting processing, a gain adjusting processing, a gammacorrection processing, a digital analog (D/A) converting processing, aformat converting processing, and so on. The picture signal processingunit 62 converts the imaging signal into a picture signal by this imageprocessing. That is, the picture signal processing unit 62 processes theimaging signal (difference signal) based on the difference calculated bythe AFE unit 51 and generates the picture signal based on the imagingsignal. The picture signal processing unit 62 outputs the generatedpicture signal to the display device 7.

The clock generating unit 63 generates a reference clock signal which isa reference for an operation of each portion of the endoscope system 1.The clock generating unit 63 outputs the generated reference clocksignal to the control signal generating unit 53.

The display device 7 displays an image captured by the imaging unit 20on the basis of the picture signal output from the picture signalprocessing unit 62. The display device 7 has a display panel such asliquid crystal or organic electro luminescence (EL).

The transmission cable 3 has a cable 31 (first coaxial cable), a cable32, a cable 33 (second coaxial cable), and a cable 34. The cable 31transmits the imaging signal output from the imaging unit 20 to theconnector unit 5. The cable 32 transmits a driving signal including thereference clock signal and the synchronization signal output from thecontrol signal generating unit 53 to the imaging unit 20. The cable 33transmits the power source voltage output from the power source unit 61to the imaging unit 20. The cable 34 transmits the ground voltage outputfrom the power source unit 61 to the imaging unit 20.

The cable 31 is a coaxial cable. FIG. 3 shows a cross section of thecable 31. As shown in FIG. 3, the cable 31 has a first conductor 310, aninsulator 311, a second conductor 312, and an outer sheath 313. Thepositions of central axes of these are the same. For example, the firstconductor 310 and the second conductor 312 are formed of metal. Forexample, the insulator 311 and the outer sheath 313 are formed ofplastic.

The first conductor 310 transmits the imaging signal output from theimaging unit 20. The insulator 311 is disposed outside the firstconductor 310 and also covers (surround) the first conductor 310. Thesecond conductor 312 is disposed outside the insulator 311 and alsocovers (surrounds) the insulator 311. The outer sheath 313 is disposedoutside the second conductor 312 and also covers (surrounds) the secondconductor 312. The first conductor 310 and the second conductor 312 areinsulated by the insulator 311. At least one of the cable 32, the cable33 and the cable 34 may be constituted with a coaxial cable.Hereinafter, an example in which the cable 32, the cable 33 and thecable 34 are constituted with coaxial cables having the same structureas that of the cable 31 will be described.

FIG. 4 shows a constitution of a main part of the endoscope system 1. InFIG. 4, the buffer 27 (voltage-current conversion circuit), thetransmission cable 3 and the circuit board 510 are shown.

The buffer 27 has a transistor M0 and a resistor R1. The resistor R1 hasa first terminal and a second terminal. The first terminal of theresistor R1 is connected to a power source VDD. The transistor M0 is aPMOS transistor. The transistor M0 has a source terminal, a drainterminal and a gate terminal. The source terminal of the transistor M0is connected to the second terminal of the resistor R1. The drainterminal of the transistor M0 is connected to the cable 31 of thetransmission cable 3. The input signal VIN is input to the gate terminalof the transistor M0. The input signal VIN is an imaging signalgenerated in the imaging unit 20 (first chip 21). The buffer 27 convertsa first voltage of the input signal VIN into a first current on thebasis of a transconductance gm of the transistor M0. A current value ofthe first current is I_(IN).

The transmission cable 3 has the cable 31, the cable 32, the cable 33and the cable 34. The first current output from the buffer 27 is inputto a first end of the first conductor 310 in the cable 31. The cable 31transmits the first current input to the first end of the firstconductor 310. The first current transmitted by the cable 31 is outputfrom a second end of the first conductor 310 in the cable 31.

The driving signal including the reference clock signal and thesynchronization signal output from the control signal generating unit 53is input to the first end of the first conductor in the cable 32. Thecable 32 transmits the driving signal input to the first end of thefirst conductor. The driving signal transmitted by the cable 32 isoutput from a second end of the first conductor in the cable 32.

The power source voltage output from the power source unit 61 is inputto the first end of the first conductor in the cable 33. The cable 33transmits the power source voltage input to the first end of the firstconductor. The power source voltage transmitted by the cable 33 isoutput from the second end of the first conductor in the cable 33.

The ground voltage output from the power source unit 61 is input to thefirst end of the first conductor in the cable 34. The cable 34 transmitsthe ground voltage input to the first end of the first conductor. Theground voltage transmitted by the cable 34 is output from the second endof the first conductor in the cable 34.

In the cable 32, the cable 33 and the cable 34, the second conductor isconnected to a ground GND. Therefore, an influence of noise due todisturbance on the signal transmitted through each cable is reduced.

The circuit board 510 has a first terminal T1, a second terminal T2 andan impedance conversion circuit 511. The first terminal T1 is connectedto the first conductor 310 of the cable 31. The second terminal T2 isconnected to the second conductor 312 of the cable 31.

The first current generated by the buffer 27 is input to the impedanceconversion circuit 511. The impedance conversion circuit 511 outputs asecond current according to the first current. The impedance conversioncircuit 511 includes a transistor M1, a current source CS1 and a currentsource CS2.

The transistor M1 is an NMOS transistor. The transistor M1 is agate-grounded transistor. The transistor M1 has a source terminal, adrain terminal and a gate terminal. The source terminal of thetransistor M1 is connected to an input terminal Tin. The drain terminalof the transistor M1 is connected to an output terminal Tout. The gateterminal of the transistor M1 is connected to a power source V1.

The current source CS1 has a first terminal and a second terminal. Thefirst terminal of the current source CS1 is connected to the inputterminal Tin. The second terminal of the current source CS1 is connectedto the ground GND. The current source CS1 is a constant current source.A current value of the current output from the current source CS1 is I₁.The current source CS2 has a first terminal and a second terminal. Thefirst terminal of the current source CS2 is connected to the powersource VDD. The second terminal of the current source CS2 is connectedto the output terminal Tout. The current source CS2 is a constantcurrent source. A current value of the current output from the currentsource CS2 is I₂. Between the power source VDD and the ground GND, thecurrent source CS1, the transistor M1 and the current source CS2 areconnected in series.

The input terminal Tin is electrically connected to the first terminalT1. The first current generated by the buffer 27 is input to the inputterminal Tin. The first current is input to the source terminal of thetransistor M1 via the input terminal Tin. A sum of the current valueI_(IN) input to the impedance conversion circuit 511, the current valueI₂ flowing in the current source CS2 and the current value I_(OUT)output from the output terminal Tout is the same as the current value I₁flowing through the current source CS1. That is, the following Equation(1) is satisfied. In FIG. 4, a direction of the current output from theoutput terminal Tout is set to a direction in which the current valueinput to the output terminal Tout becomes positive. The current valueI_(IN) and the current value I_(OUT) are different from each other.I _(IN) +I ₂ +I _(OUT) =I ₁  (1)

The impedance conversion circuit 511 outputs the second current, acurrent value of which is I_(OUT), from the output terminal Tout. Theimpedance conversion circuit 511 is a current conversion circuit havinga low input impedance and a high output impedance. Since the impedanceconversion circuit 511 outputs the second current obtained by convertingthe first current, a design of a current-voltage conversion circuit 512becomes easy.

The circuit board 510 has a signal line S1. A first end of the signalline S1 is connected to the input terminal Tin. A second end of thesignal line S1 is connected to the second terminal T2. The inputterminal Tin is electrically connected to the second terminal T2 throughthe signal line S1. A second voltage according to the first currentinput to the input terminal Tin is output from the second terminal T2.The second terminal T2 is electrically connected to the second conductor312 of the cable 31. Therefore, the second voltage output from thesecond terminal T2 is input to the second conductor 312 of the cable 31.The voltage of the first conductor 310 of the cable 31 and the voltageof the second conductor 312 are substantially the same.

The voltage of the first conductor 310 of the cable 31 changes accordingto a change in the first current. When the second conductor 312 of thecable 31 is connected to the ground GND, a parasitic capacitance betweenthe first conductor 310 and the second conductor 312 is charged anddischarged according to a change in the voltage of the first conductor310 of the cable 31. Therefore, the parasitic capacitance between thefirst conductor 310 and the second conductor 312 becomes a load of theimpedance conversion circuit 511. When a transconductance of thetransistor M1 is small, it takes a longer time to charge and dischargethe parasitic capacitance. When the transconductance of the transistorM1 is large, the time required for charging and discharging theparasitic capacitance is shortened. However, the noise of the currentgenerated by the transistor M1 increases.

In the endoscope system 1, the voltage of the first conductor 310 of thecable 31 and the voltage of the second conductor 312 are substantiallythe same. Therefore, the charging and discharging of the parasiticcapacitance between the first conductor 310 and the second conductor 312are suppressed. That is, an effect of a driven shield is obtained. As aresult, the load between the first conductor 310 and the secondconductor 312 is reduced. By reducing the load between the firstconductor 310 and the second conductor 312, the endoscope system 1 cantransmit the imaging signal at a higher speed.

The current-voltage conversion circuit 512 includes a feedback resistorR2 and an operational amplifier OP1. The feedback resistor R2 has afirst terminal and a second terminal. The operational amplifier OP1 hasa non-inverting input terminal, an inverting input terminal, and anoutput terminal. A first terminal of the feedback resistor R2 isconnected to the inverting input terminal of the operational amplifierOP1. A second terminal of the feedback resistor R2 is connected to theoutput terminal of the operational amplifier OP1. The inverting inputterminal of the operational amplifier OP1 is connected to the outputterminal Tout of the impedance conversion circuit 511. The non-invertinginput terminal of the operational amplifier OP1 is connected to a powersource that outputs a reference voltage VREF.

The second current output from the impedance conversion circuit 511 isinput to the current-voltage conversion circuit 512. The current-voltageconversion circuit 512 converts the second current output from theimpedance conversion circuit 511 into a third voltage. Thecurrent-voltage conversion circuit 512 outputs the third voltage as anoutput signal VOUT from an output terminal of the operational amplifierOP1.

A voltage value V_(OUT) of the output signal VOUT is expressed byEquation (2). In Equation (2), V_(REF) is a voltage value of thereference voltage of the current-voltage conversion circuit 512. InEquation (2), R is a resistance value of the feedback resistor R2.V _(OUT) =V _(REF) +R×I _(OUT)  (2)

As described above, the impedance conversion circuit 511 and thecurrent-voltage conversion circuit 512 are disposed on the circuit board510. A circuit for calculating a difference between the reference signaland the imaging signal is disposed at a subsequent stage of the circuitboard 510. The circuit board 510 may be formed separately from the AFEunit 51 and may be disposed at a preceding stage of the AFE unit 51.

When the first current is transmitted in an ideal current mode by thecoaxial cable and also the transmitted first current is directly inputto the current-voltage conversion circuit 512, the voltage of theinverting input terminal of the operational amplifier OP1 is keptsubstantially constant. Therefore, it is not necessary for the secondvoltage according to the first current to be input to the secondconductor 312. However, the first current is input to the impedanceconversion circuit 511, and the input impedance of the impedanceconversion circuit 511 is not zero. Accordingly, the voltage of theinput terminal Tin changes due to the change of the first current. Toreduce an influence of charging and discharging of the parasiticcapacitance due to this voltage change, the second voltage according tothe first current input to the input terminal Tin is input to the secondconductor 312 of the cable 31.

A conductivity type of each transistor used for the buffer 27 and theimpedance conversion circuit 511 may be opposite to the above-describedconductivity type. In the buffer 27 and the impedance conversion circuit511, although metal oxide semiconductor (MOS) transistors are used,bipolar transistors may be used.

The endoscope system according to each aspect of the present inventionneed not have the constitution corresponding to at least one of theoperation unit 4, the processor 6 and the display device 7. Theendoscope system according to each aspect of the present invention neednot have the constitution corresponding to at least one of thepreprocessing unit 52 and the control signal generating unit 53. Theendoscope system according to each aspect of the present invention neednot have the constitution corresponding to at least one of the cable 32,the cable 33 and the cable 34. The endoscope system according to eachaspect of the present invention need not have the current-voltageconversion circuit 512.

The imaging signal is not limited to an analog signal. The imagingsignal may be a digital signal. When the imaging signal is a digitalsignal, the connector unit 5 includes a circuit such as a comparatorcapable of detecting a High/Low voltage instead of the AFE unit 51.

As described above, the endoscope system 1 includes the imaging unit 20(imaging element), the buffer 27 (voltage-current conversion circuit),the cable 31 (first coaxial cable) and the impedance conversion circuit511. The imaging unit 20 generates the first voltage (imaging signal).The buffer 27 is disposed inside or outside the imaging unit 20 andconverts the first voltage into the first current. The cable 31 has thefirst conductor 310 and the second conductor 312. The second conductor312 is disposed outside the first conductor 310. The first current istransmitted through the first conductor 310. The first currenttransmitted by the first conductor 310 is input to the impedanceconversion circuit 511. The impedance conversion circuit 511 outputs asecond current according to the first current. The second voltageaccording to the first current is input to the second conductor 312.

In the first embodiment, the second voltage according to the firstcurrent is input to the second conductor 312. Therefore, the endoscopesystem 1 can transmit a signal from the imaging unit 20 at a higherspeed.

The endoscope system 1 may include the cable 33 (second coaxial cable)which transmits a power source voltage supplied to the imaging unit 20(imaging element). By inputting the second voltage according to thefirst current into the second conductor 312, the parasitic capacitanceis formed between the second conductor 312 and the cable 33. Due to thechange in the second voltage, noise may be transmitted to the cable 33via the parasitic capacitance. Since the cable 33 is constituted with acoaxial cable, it is difficult for the noise to be transmitted to thepower source voltage transmitted through the cable 33. The cable 32 andthe cable 34 are also formed of coaxial cables. Accordingly, it isdifficult for the noise to be transmitted to the drive signaltransmitted through the cable 32 and the ground voltage transmittedthrough the cable 34.

Modified Example of First Embodiment

FIG. 5 shows a constitution of a main part of the endoscope system 1 aaccording to a modified example of the first embodiment. In FIG. 5, abuffer 27 (voltage-current conversion circuit), a transmission cable 3and a circuit board 510 a are shown. With respect to the constitutionshown in FIG. 5, points different from the constitution shown in FIG. 4will be described.

The circuit board 510 shown in FIG. 4 is changed to the circuit board510 a shown in FIG. 5. In the circuit board 510 a, the impedanceconversion circuit 511 shown in FIG. 4 is changed to an impedanceconversion circuit 511 a, and the current-voltage conversion circuit 512shown in FIG. 4 is changed to a current-voltage conversion circuit 512a.

In the impedance conversion circuit 511 a, the current source CS2 in theimpedance conversion circuit 511 is deleted.

The current-voltage conversion circuit 512 a has a resistor R3. Theresistor R3 has a first terminal and a second terminal. The firstterminal of the resistor R3 is connected to an output terminal Tout ofthe impedance conversion circuit 511 a. The second terminal of theresistor R3 is connected to a power source VDD.

A second current output from the impedance conversion circuit 511 a isinput to the current-voltage conversion circuit 512 a. Thecurrent-voltage conversion circuit 512 a converts the second currentinto a third voltage and outputs the third voltage as an output signalVOUT from the first terminal of the resistor R3.

Regarding points other than the above, the constitution shown in FIG. 5is the same as the constitution shown in FIG. 4. Constitutions otherthan the constitution shown in FIG. 5 are the same as those which havealready been described.

The impedance conversion circuit 511 shown in FIG. 4 and thecurrent-voltage conversion circuit 512 a shown in FIG. 5 may be disposedon the circuit board. The impedance conversion circuit 511 a shown inFIG. 5 and the current-voltage conversion circuit 512 shown in FIG. 4may be disposed on the circuit board.

Second Embodiment

FIG. 6 shows the constitution of a main part of an endoscope system 1 bof a second embodiment. In FIG. 6, a buffer 27 (voltage-currentconversion circuit), a transmission cable 3 and a circuit board 510 bare shown. In the constitution shown in FIG. 6, points different fromthe constitution shown in FIG. 4 will be described.

The circuit board 510 shown in FIG. 4 is changed to a circuit board 510b shown in FIG. 6. The circuit board 510 b includes an impedanceconversion circuit 511, a current-voltage conversion circuit 512 and abuffer 513.

The buffer 513 has an input terminal and an output terminal. The inputterminal of the buffer 513 is connected to an input terminal Tin of theimpedance conversion circuit 511. Therefore, the second voltageaccording to the first current is input to the buffer 513. The outputterminal of the buffer 513 is connected to a second terminal T2.Therefore, the buffer 513 outputs the second voltage to the secondterminal T2.

For example, the buffer 513 is a voltage follower. The buffer 513 may bea source follower using a MOS transistor. The buffer 513 may be anemitter follower using a bipolar transistor. The buffer 513 may be acircuit other than these circuits. It is relatively easy to reduce anoutput impedance of the buffer 513. Therefore, a shielding effectagainst the noise due to disturbance can be sufficiently secured.

Regarding points other than the above, the constitution shown in FIG. 6is the same as the constitution shown in FIG. 4. Constitutions otherthan the constitution shown in FIG. 6 are the same as those which havealready been described. The circuit board 510 a shown in FIG. 5 may havethe buffer 513.

As described above, the endoscope system 1 b has the buffer 513 whichoutputs the second voltage according to the first current. By disposingthe buffer 513 having a driving capability, the transconductance of thetransistor M1 can be reduced.

The circuit board 510 in the first embodiment has the first terminal T1and the second terminal T2. The first current transmitted by the firstconductor 310 of the cable 31 is input to the first terminal T1. Thefirst terminal T1 is electrically connected to the impedance conversioncircuit 511. The second terminal T2 is electrically connected to theimpedance conversion circuit 511 and also outputs the second voltage.

By providing the first terminal T1 and the second terminal T2, itbecomes easy to insert the buffer 513 between the input terminal Tin andthe second terminal T2 of the impedance conversion circuit 511.

Third Embodiment

FIG. 7 shows a constitution of the main part of an endoscope system 1 caccording to a third embodiment. In FIG. 7, a buffer 27 (voltage-currentconversion circuit), a transmission cable 3 and a circuit board 510 care shown. Regarding the constitution shown in FIG. 7, points differentfrom the constitution shown in FIG. 4 will be described.

The circuit board 510 shown in FIG. 4 is changed to a circuit board 510c shown in FIG. 7. In the circuit board 510 c, the impedance conversioncircuit 511 shown in FIG. 4 is changed to an impedance conversioncircuit 511 c.

The impedance conversion circuit 511 c includes a current source CS1, acurrent source CS2, a transistor M2 and a transistor M3.

The current source CS1 has a first terminal and a second terminal. Thefirst terminal of the current source CS1 is connected to the powersource VDD. The second terminal of the current source CS1 is connectedto the input terminal Tin. The current source CS1 is a constant currentsource. A current value of a current output from the current source CS1is I₁. The current source CS2 has a first terminal and a secondterminal. The first terminal of the current source CS2 is connected tothe power source VDD. The second terminal of the current source CS2 isconnected to the output terminal Tout. The current source CS2 is aconstant current source. A current value of a current output from thecurrent source CS2 is I₂.

The transistor M2 and the transistor M3 constitute a current mirror. Thetransistors M2 and M3 are NMOS transistors. The transistor M2 and thetransistor M3 have a source terminal, a drain terminal and a gateterminal. The drain terminal of the transistor M2 is connected to theinput terminal Tin. The source terminal of the transistor M2 isconnected to the ground GND. The gate terminal of the transistor M2 isconnected to the drain terminal of the transistor M2. The drain terminalof the transistor M3 is connected to the output terminal Tout. Thesource terminal of the transistor M3 is connected to the ground GND. Thegate terminal of the transistor M3 is connected to the gate terminal ofthe transistor M2. The transistor M3 is connected to the current-voltageconversion circuit 512 via the output terminal Tout. Between the powersource VDD and the ground GND, the current source CS1 and the transistorM2 are connected in series and the current source CS2 and the transistorM3 are also connected in series.

A first current generated by the buffer 27 is input to the inputterminal Tin. The first current is input to the transistor M2 via theinput terminal Tin. The first current flows between the drain terminaland the source terminal of the transistor M2. A current according to amirror ratio between the transistor M2 and the transistor M3 flowsbetween the drain terminal and the source terminal of the transistor M3.It is assumed that W/L ratios of the transistor M2 and the transistor M3are the same. When a coefficient of the transistor M2 is m and acoefficient of the transistor M3 is n, the current value of the currentflowing through the transistor M3 is (n/nm) times the current value ofthe current flowing through the transistor M2. When the coefficients ofthe transistor M2 and the transistor M3 are the same, the currentsflowing through the transistor M2 and the transistor M3 are the same.The impedance conversion circuit 511 c outputs a second current, acurrent value of which is I_(OUT), from the output terminal Tout. Theimpedance conversion circuit 511 c is a current conversion circuithaving a low input impedance and a high output impedance.

The input terminal Tin is electrically connected to the first terminalT1. A first current generated by the buffer 27 is input to the inputterminal Tin. The first current is input to the transistor M2 via theinput terminal Tin. For ease of explanation, it is assumed that themirror ratio between the transistor M2 and the transistor M3 is 1. Thecurrent value of the current flowing through the transistor M2 isI_(M2). The transistor M3 outputs a current having the same currentvalue as that of the current flowing through the transistor M2. Thecurrent value of the current flowing through the transistor M3 isI_(M3).

A sum of the current value I_(IN) input to the impedance conversioncircuit 511 c and the current value I₁ flowing through the currentsource CS1 is the same as the current value I_(M2) flowing through thetransistor M2. That is. Equation (3) is satisfied.I _(IN) +I ₁ =I _(M2)  (3)

A sum of the current value I₂ flowing through the current source CS2 andthe current value I_(OUT) output from the output terminal Tout is thesame as the current value I_(M3) flowing through the transistor M3. Thatis, Equation (4) is satisfied. In FIG. 7, a direction of the currentoutput from the output terminal Tout is set in a direction in which thecurrent value input to the output terminal Tout becomes positive.I ₂ +I _(OUT) =I _(M3)  (4)

Regarding points other than the above, the constitution shown in FIG. 7is the same as the constitution shown in FIG. 4. Constitutions otherthan the constitution shown in FIG. 7 are the same as those which havealready been described. The impedance conversion circuit 511 c may havethe buffer 513 shown in FIG. 6.

The conductivity type of each transistor used for the buffer 27 and theimpedance conversion circuit 511 c may be opposite to theabove-described conductivity type. In the buffer 27 and the impedanceconversion circuit 511 c, although MOS transistors are used, bipolartransistors may be used.

In the third embodiment, the second voltage according to the firstcurrent is input to the second conductor 312. Therefore, the endoscopesystem 1 c can transmit a signal from the imaging unit 20 at a higherspeed.

Modified Example of Third Embodiment

FIG. 8 shows a constitution of a main part of an endoscope system 1 daccording to a modified example of the third embodiment. In FIG. 8, abuffer 27 (voltage-current conversion circuit), a transmission cable 3and a circuit board 510 d are shown. Regarding the constitution shown inFIG. 8, points different from the configuration shown in FIG. 7 will bedescribed.

The circuit board 510 c shown in FIG. 7 is changed to a circuit board510 d shown in FIG. 8. In the circuit board 510 d, the impedanceconversion circuit 511 c shown in FIG. 7 is changed to an impedanceconversion circuit 511 d, and the current-voltage conversion circuit 512shown in FIG. 7 is changed to a current-voltage conversion circuit 512a.

In the impedance conversion circuit 511 d, the current source CS2 in theimpedance conversion circuit 511 c is deleted. The current-voltageconversion circuit 512 a is the same as the current-voltage conversioncircuit 512 a shown in FIG. 5. The same current as the current flowingthrough the transistor M3 is output as the second current from theoutput terminal Tout.

The second current output from the impedance conversion circuit 511 d isinput to the current-voltage conversion circuit 512 a. Thecurrent-voltage conversion circuit 512 a converts the second currentinto a third voltage and outputs the third voltage as an output signalVOUT from the first terminal of the resistor R3.

Regarding the points other than the above, the constitution shown inFIG. 8 is the same as the constitution shown in FIG. 7. Constitutionsother than the constitution shown in FIG. 8 are the same as those whichhave already been described. The impedance conversion circuit 511 d mayhave the buffer 513 shown in FIG. 6.

The impedance conversion circuit 511 c shown in FIG. 7 and thecurrent-voltage conversion circuit 512 a shown in FIG. 8 may be disposedon the circuit board. The impedance conversion circuit 511 d shown inFIG. 8 and the current-voltage conversion circuit 512 shown in FIG. 7may be disposed on the circuit board.

While preferred embodiments of the invention have been described andshown above, it should be understood that these are exemplary of theinvention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

What is claimed is:
 1. An endoscope system, comprising: an imagingelement configured to generate a first voltage, a voltage-currentconversion circuit disposed inside or outside the imaging element andconfigured to convert the first voltage into a first current, a firstcoaxial cable having a first conductor and a second conductor, thesecond conductor being disposed outside the first conductor, the firstcurrent being transmitted through the first conductor, a current-voltageconversion circuit configured to convert the second current output fromthe impedance conversion circuit into a third voltage, and a circuitboard on which the impedance conversion circuit and the current-voltageconversion circuit are disposed, an impedance conversion circuit towhich the first current transmitted through the first conductor is inputand configured to output a second current according to the firstcurrent, wherein a second voltage according to the first current isinput to the second conductor, and the circuit board comprises a firstterminal to which the first current transmitted through the firstconductor is input and which is electrically connected to the impedanceconversion circuit, and a second terminal electrically connected to theimpedance conversion circuit and configured to output the secondvoltage.
 2. The endoscope system according to claim 1, furthercomprising a buffer configured to output the second voltage according tothe first current.
 3. The endoscope system according to claim 1, furthercomprising a second coaxial cable configured to transmit a power sourcevoltage supplied to the imaging element.